64 Geophysics in Geothermal Exploration The density contrast leads to a different gravitational force which is measured, and usually presented in mgal or 10–3 cm/s2. To obtain information about the subsurface density from the gravity measurement, it is necessary to make several corrections to the measured value. The final corrected values of the gravity anomaly, is called Bouguer anomaly and is given by: Δg = gobs – gϕ + Δgal – ΔgBoug +TC (2.9) where gobs are gravity readings observed at each gravity station after corrections have been applied for instrument drift and earth tides. gϕ is the gravity at latitude ϕ. Δgal = 0.3086h is the Free air correction or elevation correction to consider the variation of elevation h between the measurement locations (a vertical decrease of gravity is associated with an increase of elevation). ΔgBoug = 0.042ρh is the correction from the excess mass material between the station and sea level. (variation of elevation h, density ρ). TC is the terrain correction that accounts for the presence of mass (for example hill) in the vicinity of the measurement station. Bouguer anomalies are differences between actual gravity values and what would be expected from a uniform earth, which has the same latitude, elevation, and terrain. Gravity interpretation frequently begins with some procedure that separates the anomalies of interest from the smooth presumably deep regional effects. The regional effect could be obtained by a filtering process, such as upward continuation intended to emphasize or enhance the long wavelength components. The regional is subtracted from the observed gravity map or profile, and the resulting residual contains the component of the field which are caused by mass representing geological disturbances of interest. Figure 2.7 is an example of Bouguer anomaly observed in Martinique (Girard, 2017). The studied area is located close to the site called “Anses d’Arlets”, south-West of Martinique (Figure 2.7a). Geology is mainly composed of andesitic to dacitic products with basaltic lavas (age ranges from 1.5 to 0.35 million years). The density variation in the ground affects the gravity recorded in surface, and after some corrections have been applied, a Bouguer anomaly map can be obtained. The average density which decorrelates the more the topographic effect (Nettleton, 1939) is close to 2, and was used to compute the map shown in Figure 2.7b. A high pass filter was applied (cut length 8 km). Black dots are gravimetric measurements. The U.S. Navy has developed a system to measure gravity gradients. In 1994, this technology began to be used in exploration (Bell et al., 1997). Normal gravity field gz, also noted Gz, is the vertical pull of gravity at a location and is made up of two signals: one from the local geology and the other from the best-fitting ellipsoid field (Fairhead, 2015). In airborne gravity, the airborne gravimeter measures the sum (gz + a) of the vertical acceleration “a” and of the normal gravity field gz. To recover the normal gravity field, the vertical acceleration must be canceled using additional information given by GPS measurements. The measurement of gz at two elevations z + Δz/2 and z – Δz/2 leads to the cancelation of the kinematic acceleration by computing the first vertical derivative Gzz of the gravity field. Such an approach is known as Gravity-gradiometry, the measurements being done by gravity gradiometers.
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